1. Field of the Invention
The present invention is related to a ghost canceler and, more specifically, to a ghost canceler cancelling ghost signals which adaptively amplifies a video signal, the signal level of which is varied according to ghost phenomenon generated in the transmission of the video signal.
Korean Patent Application No. 92-25962 is incorporated herein by reference for all purposes.
2. Description of the Prior Art
The ghost phenomenon, that is, the phenomenon of displaying ghost images on a television screen occurs due to the reflectance of the radio frequency waves from the broadcasting station off of mountains or buildings. To cancel such ghost phenomenon, the broadcasting station transmits a ghost cancelling reference (GCR) signal so that the features of the transmission channel can be seen at the receiver. Accordingly, the GCR signal is used in the receiver to determine the features of the ghost in the transmission channel and to cancel the ghost signal from the transmitted video signal. Such technology for cancelling the ghost using the GCR signal has been widely used throughout the world, e.g., in Japan, U.S.A., Korea, etc.
FIG. 1 is a block diagram of the conventional ghost canceler, wherein an analog-to-digital (A/D) converter 15 is connected to an input end IN. The whole system is controlled by a microprocessor 11. A ROM 22, which stores a GCR signal, is connected to one input of microprocessor 11. A ghost cancelling circuit 17 comprises a plurality of finite impulse response (FIR) filters and infinite impulse response (IIR) filters, and performs signal processing on the output data from A/D converter 15 according to a control signal from microprocessor 11. The data output end of ghost cancelling circuit 17 is connected to a digital-to-analog (D/A) converter 18. A GCR detector 20 is connected to input end IN, the output of A/D converter 15 and another input of microprocessor 11, and detects the received GCR signal according to another control signal of microprocessor 11.
GCR signal detector 20 comprises a sync and field separator 14 which is connected to signal input end IN for detecting a horizontal sync signal H, a vertical sync signal and a field identifier FID, a first-in-first-out (FIFO) memory 16 which stores a GCR signal among the output data of A/D converter 15 and supplies the stored GCR signal to microprocessor 11, and a window pulse generator 13 for receiving the output signals H, V and FID of sync and field separator 14 and the control signal of microprocessor 11 and generating a window signal for detecting the GCR signal from the received video signal.
FIGS. 2A through 2C show waveform diagrams of the video signal of which the signal level is varied according to the ghost. FIG. 2A shows an original signal without any ghost, FIG. 2B shows a ghost signal having a predetermined delay time T.sub.d and a predetermined amplitude AM with respect to the original signal, and FIG. 2C shows a video signal including a ghost signal. Since the generally received video signal includes the ghost, the conventional ghost canceler for cancelling the ghost of -6 dB has a bit resolution capable of processing a signal of which the amplitude becomes larger by one-half with respect to the video signal without the ghost. In this manner, A/D converter 15 is designed to have the bit resolution which represents the number of bits which are assigned to the actual data.
Microprocessor 11 reads line information having the GCR signal from ROM 22 and generates the read line information for window pulse generator 13. If the video signal including the ghost is input to the ghost canceler through input end IN, sync and field separator 14 detects horizontal sync H, vertical sync V and field identifier FID from the input video signal and supplies the detected signal H, V and FID to window pulse generator 13. The detected field identifier FID is for discriminating an odd field and an even field. A/D converter 15 converts the input video signal into digital video data and supplies the digital video data to FIFO memory 16 and ghost cancelling circuit 17. Window pulse generator 13 receives output signals H, V and FID of sync and field separator 14 and the GCR line information of microprocessor 11 and generates a window pulse. The FIFO memory 16 stores the GCR signal which is included in the video signal according to the window pulse. When the GCR signal is stored in FIFO memory 16 according to the window pulse, microprocessor 11 reads the GCR signal from FIFO memory 16 and compares the read GCR signal with preset GCR signal provided from ROM 22, to produce a filter coefficient.
Ghost cancelling circuit 17 cancels the ghost from the video signal using the filter coefficient and a number of incorporated impulse response filters. A digital-to-analog (D/A) converter 18 converts the ghost-cancelled video data into an analog signal and outputs the converted analog signal.
As described above, the conventional ghost canceler digitizes the video signal including the ghost without performing amplification control to cancel the ghost. Accordingly, the degree of information loss is varied due to analog-to-digital conversion according to the magnitude of the ghost signal which is input to the A/D converter. As a result, not only is the performance of the ghost canceler is degraded, but also the quality of the displayed image picture is lowered.